A trend in digital camera systems has been the shrinking of both optical elements and detector arrays. As these elements shrink, however, the optical performance of their optical systems suffer since the angular resolution and number of resolvable object points both decrease as the diameter of the imaging system is reduced.
A digital imaging system provides light from an object or scene to an opto-electronic detector array. Each detector provides an electrical signal whose magnitude is a function of the intensity of the light incident on that detector. As a result, the detector array enables the light to be electronically processed in order to estimate properties of the object. Typically, it is desirable for the lens system to (1) collect as much of the light signal as possible over as large an aperture as possible; and (2) process the collected light signal to either form an optical image on the detector array or to encode the light signal for digital image estimation.
Lens system design begins by specifying targets for major performance metrics, such as angular resolution, field-of-view, depth of field, spectral range, sensitivity, dynamic range, system mass and volume. Angular resolution is generally the most significant initial metric. The best angular resolution of a lens is given by λ/A, where λ is the operating wavelength and A is the collection aperture diameter. Once the collection aperture size has been determined by this relationship, a lens is designed to achieve the remaining performance metrics by judicious choice of materials and surface profiles.
In conventional lens design, the aperture size of an entrance lens or optical stop (i.e., the primary aperture) often determines the effective aperture size of all subsequent lens surfaces (i.e., the secondary aperture) in the lens system. The use of multiple lenses and apertures enables a lens system to simultaneously: (1) create an effective focal length and magnification appropriate to the imaging task at hand; (2) reduce image aberrations; and (3) provide correct image orientation. Secondary apertures are typically matched to the effective cross section of the magnified or demagnified entrance aperture propagated through the lens system. In systems with low aberration, the size of the entrance aperture often determines angular resolution of the lens system while the size of the secondary apertures determines the field-of-view of the lens system.
Simple cameras typically balance field-of-view and resolution by using a sequence of lenses having approximately equally sized apertures. Microscopes, on the other hand, achieve large field-of-view and high angular resolution by increasing secondary aperture relative to the collection aperture. Telescopes achieve extra-ordinary angular resolution with a limited field-of-view by decreasing secondary aperture size. Wide-field cameras achieve large field-of-view by tolerating significant aberration across the image with approximately equal primary and secondary apertures. Conventional lens design, therefore, normally requires trade-offs between desired performance metrics. For example, telescopes achieve high angular resolution by sacrificing field-of-view, wide-field imagers achieve large angular fields-of-view by sacrificing diffraction-limited angular resolution, and compound-optics cameras achieve high quality by expanding system volume to include more aberration-correction optics.
In order to overcome some of the limitations of standard imaging optics, multi-aperture cameras have been developed. In multi-aperture systems, a standard camera objective lens is replaced by an array of lenslets, wherein each lenslet has a reduced focal length in comparison to a conventional camera. In such approaches, a detector measures a set of sub-sampled versions of the object within the field-of-view. Post-processing algorithms are used to generate a high-resolution image from the set of sub-sampled sub-images. The result is reduced system volume; however, the reduction in system volume is achieved at the cost of significant computational post-processing and compromised image quality.
In addition, the design space for multi-aperture cameras is severely restricted, which has limited their adoption in practical systems. The use of a multi-aperture camera requires that the size of its detector array and system aperture be approximately the same size. As a result, conventional multi-aperture designs are generally restricted to very small collection apertures. This also limits the number of camera formats that can be designed. Further, a multi-aperture camera typically has a restricted field-of-view due to a need to prevent the overlapping of sub-images on the detector array. Such overlapping can be avoided by introducing a field stop in the optical design; however, this increases system volume. Alternatively, absorbing barriers can be placed between the sub-image regions of the detector array; however, this significantly increases manufacturing cost and complexity.
A lens system that avoids some of the design trade-offs associated with conventional lens design and that achieves high performance cost-effectively is desirable.